The present invention pertains to method and apparatus for measuring optical quality of an eye. In particular, the present invention pertains to method and apparatus which measures refractive errors of an eye based on wavefront measurement.
As is well known, a refractor is an optical apparatus used to measure refractive errors of an eye. In particular, a wavefront refractor is a refractor based on wavefront measurement, and as is further well known, a Hartmann-Shack wavefront sensor can be used to construct such a wavefront refractor.
FIG. 2 shows a block diagram of prior art wavefront refractor 200. As shown in FIG. 2, wavefront refractor 200 comprises probe beam assembly 10, polarizing beamsplitter 20, relay optics assembly 40, and Hartmann-Shack sensor assembly 50xe2x80x2. As shown in FIG. 2, probe beam assembly 10 comprises a radiation source (not shown) which outputs beam of radiation 11, which beam of radiation 11 (after being redirected by turning reflector 12) is applied as input to polarizing beamsplitter 20. Beam of radiation 11 typically comprises radiation that is not detected by a patient such as, for example, infrared or near infrared radiation. The source of beam of radiation 11 may be a super-luminescent diode or a laser. The beam of radiation output from polarizing beamsplitter 20 is directed to impinge upon eye 30 to form illumination spot 32 on retina 31.
As shown in FIG. 2, radiation scattered from illumination spot 32 passes through the eye""s optics (including eye lens 34 and cornea 35), and emerges as outgoing beam 33. The wavefront of outgoing beam 33 carries aberration information directly relating to the optical quality of the eye""s optics. For example, for a perfect emmetropic eye without aberration error, the wavefront of outgoing beam 33 is a flat plane; for a myopic or hyperopic eye, the wavefront of outgoing beam 33 has the shape of a spherical surface; and for an eye with high order aberrations, the wavefront of outgoing beam 33 is distorted irregularly.
As shown in FIG. 2, relay optics assembly 40 relays the wavefront of outgoing beam 33 from exit pupil plane P of eye 30 to Hartmann-Shack sensor assembly 50xe2x80x2 disposed at conjugate plane Pxe2x80x2. As further shown in FIG. 2, Hartmann-Shack sensor assembly 50xe2x80x2 of prior art wavefront refractor 200 comprises lenslet array 51 and CCD camera 53. The principles and design parameters used to fabricate Hartmann-Shack sensor assembly 50xe2x80x2 are well known in the art. In accordance with the prior art design, CCD camera 53 is located at the focal plane of the lenslet elements of lenslet array 51, and prior art Hartmann-Shack sensor assembly 50xe2x80x2 detects the wavefront of outgoing beam 33 when lenslet array 51 divides the wavefront of outgoing beam 33 into sub-apertures of the lenslets. Each lenslet forms a focal spot such as focal spot 52 on CCD camera 53, and as is well known, the pattern of focal spots carries the signature of the wavefront of the beam to be measured.
In accordance with this prior art design, output from CCD camera 53 is applied as input to analyzer 60, for example, a personal computer. Analyzer 60 then determines the x, y, z position of a centroid of each of the focal spots in accordance with any one of a number of methods that are well known to those of ordinary skill in the art. Next, analyzer 60 determines the slope of each beam segment using the coordinates of the centroids to determine the slope of a portion of the beam passing through each of the elements of lenslet array 50. Next, analyzer 60 uses any one of a number of methods that are well known to those of ordinary skill in the art to use the slopes of the beam segments to reconstruct the wavefront of beam 33 at plane Pxe2x80x2. For example, in one such embodiment, analyzer 60 fits the slopes of the beam segments to a set of Zernike polynomials to reconstruct the wavefront of beam 33 at plane Pxe2x80x2 in accordance with the teaching of an article entitled xe2x80x9cObjective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensorxe2x80x9d by J. Liang et al., J. Opt. Soc. Am. A, Vol. 11, No. 7, July 1994, pp. 1949-1957, and an article entitled xe2x80x9cAberrations and retinal image quality of the normal human eyexe2x80x9d by J. Liang et al., J. Opt. Soc. Am. A, Vol. 14, No. 11, November 1997, pp. 2873-2883 (the xe2x80x9cLiang articlesxe2x80x9d), which Liang articles are incorporated by reference herein. The wavefront of outgoing beam 33 is then reconstructed at plane P via a scale factor determined by relay optics assembly 40. A review of the Hartmann-Shack wavefront sensor, and wavefront reconstruction is found in U.S. Pat. No. 5,777,719. Finally, the refractive errors of the eye are calculated by analyzer 60 in accordance with any one of a number of methods that are well known to those of ordinary skill in the art using the reconstructed wavefront. For example, one such method is disclosed in a publication of Frey et al. on Jun. 3, 1999, WO 99/27334 entitled xe2x80x9cObjective Measurement and Correction of Optical Systems Using Wavefront Analysisxe2x80x9d wherein distortions of the wavefront are taken as an estimate of the aberrations, which publication is incorporated by reference herein (see also the Liang articles). An algorithm for use in analyzer 60, for example, a computer algorithm, is commercially available from, for example, Adaptive Optics Associates of Cambridge, Mass.
Comprehensive measurement of the refractive errors of the eye""s optics provided by prior art wavefront refractor 200 include high order aberrations. Such comprehensive measurement of the refractive errors can be used to guide laser surgery to correct the refractive errors of the eye. Advantageously, a wavefront refractor can provide more accurate measurement of the refractive errors of an eye than a conventional auto-refractor. Consequently, a wavefront refractor may eventually be used to provide prescriptions for eyeglasses and contact lenses.
However, several problems arise in using prior art wavefront refractor 200 which includes Hartmann-Shack sensor assembly 200. A first problem arises because refractive errors of a human eye can be substantial, and as a result, wavefront distortion can be significant. For the case of a planar wavefront produced by an emmetropic eye, focal spots 52 form a grid pattern on CCD camera 53 that is identical to that of lenslet array 51. However, for a case where wavefront distortion is significant, the grid pattern of focal spots 52 may be badly distorted. This causes a need for an algorithm (used to analyze the grid pattern of focal spots) to associate each focal spot with the particular lenslet used to form the focal spot.
A second problem arises because distortion of the grid pattern of focal spots can arise from a source other than refractive error of the eye. In particular, the intensity distribution of the outgoing beam can vary significantly across the exit pupil due to a number of effects, including, for example, the scattering nature of the probe beam from the retina. In such a case, some focal spots (referred to herein as xe2x80x9cbadxe2x80x9d spots) may have their centroid shifted away from their chief raysxe2x80x94such a shift occurs whenever the intensity distribution across a lenslet has a strong slope. This causes a need for an algorithm (used to analyze the grid pattern of focal spots) to reject these shifted or xe2x80x9cbadxe2x80x9d spots.
A third problem arises because unwanted trace beams, i.e., beams reflected, for example, from optics elements and/or the eye, can not always be removed. In such a case, xe2x80x9cghostxe2x80x9d focal spots may appear in the grid pattern of focal spots. This causes a need for an algorithm (used to analyze the grid pattern of focal spots) to identify these ghost spots.
U.S. Pat. No. 5,629,765 (the ""765 patent) discloses a spot matching technique that moves a CCD camera longitudinally, and records images in several different positions. The images are then analyzed, and the displacement of each focal spot is traced toward a lenslet. By matching each focal spot to a lenslet, the average tilt of the test beam can be determined, and the sub-aperture of each lenslet can be realigned on the CCD camera. This technique enables accurate and reliable measurement of a test beam with large overall tilt with respect to an instrument axis. However, this does not solve any of the above-identified problems with respect to measurement of refractive errors of an eye. In addition to this, camera movement is typically much slower than eye movement. As a result, a movable camera does not provide a practical solution to the problems that occur in measuring refractive errors of an eye.
Embodiments of the present invention advantageously satisfy the above-identified problems in the art, and provide method and apparatus for measuring refractive errors of an eye. Advantageously, in accordance with the present invention, embodiments of the present invention enable: (a) measurement of an eye with large refractive errors; (b) measurement of a test beam with large wavefront distortion; (c) rejection of xe2x80x9cbadxe2x80x9d spots and xe2x80x9cghostxe2x80x9d spots; and (d) reliable measurement which is free from eye movement.
Specifically, one embodiment of the present invention is a wavefront refractor which comprises: (a) relay optics adapted to relay a wavefront of a beam emerging from an eye from a corneal plane to a conjugate plane; (b) a lenslet array disposed at the conjugate plane to intercept the relayed beam; (c) a beamsplitter disposed behind the lenslet array to split radiation transmitted by the lenslet array into first radiation and second radiation; (d) a first camera disposed to receive the first radiation at a distance from the lenslet array that is shorter than a focal length of a lenslet; and (e) a second camera disposed to receive the second radiation at a distance from the lenslet array that is longer than a focal length of the lenslet.